This invention relates generally to an offshore platform such as those used in oil exploration and production operations. More specifically, the invention concerns an improved jacking assembly for use with those platforms.
Jackup barges and platforms for use in offshore oil exploration and offshore oil production facilities have long been known. Generally, these platforms are floated into position and slidable legs are extended from the platform to the seabed. With continued extension, the legs ultimately reach a point where the resistance to further penetration of the legs into the seabed exceeds the total weight of the barge. At this time, further attempted relative movement between the legs and the platform causes the platform to be lifted vertically out of the water to an elevated position.
It has been found that the period of time during which the platform moves from a buoyant or floating position to the elevated position, where the platform is a static weight supported solely by the legs engaging the seabed is a particularly important time insofar as the jackup procedure is concerned. During this period of time, the action of wave forces on the platform can exert substantial lateral forces on the legs which, by virtue of their generally open framework, are not ordinarily adversely affected by wave action. This lateral force coupled with the massive inertia of the platform relative to the legs makes it desirable to move through this transition period of wave-platform interaction as briskly as possible.
The currently accepted approach universally used in moving through this transition period provides a gear rack which extends along the entire length of the leg from the top to the bottom, a distance which may be 400 to 500 feet. This gear rack is engaged by a suitable driven pinion which moves the leg relative to the platform. Realizing that each leg is likely to be provided with three or more of such gear racks each fashioned from high tensile strength flame-cut steel, it is apparent that a substantial economic factor is involved in the construction of a jackup platform. Jacking devices of the above described type are fabricated by Marathon-LeFourneau Offshore Company, among others.
Another type of jacking assembly used in offshore platforms includes a yoke selectively engageable with the leg and means for selectively affixing the leg relative to the platform. In one such device (see for example, U.S. Pat. No. 3,517,910 issued to Sutton et al on June 30, 1970), the jacking mechanism includes a ball screw and cooperating shaft to move the yoke relative to the platform. While this jacking mechanism is extremely efficient in terms of the manner in which it lifts a platform, ball screws and ball nuts in the large diameter sizes required for platform lifting operations are expensive and not yet commonly used in the oil exploration and oil production arts. Accordingly, even this type of device is capable of further refinement.
In another version of a jacking assembly, a relatively long ball screw and cooperating ball nut were contemplated in a jackup offshore platform, see U.S. Pat. No. 3,282,565, issued to Sutton on Nov. 1, 1966. That jacking assembly was selected to overcome shortcomings of hydraulic cylinders and provide an alternative to continuously operable rack and pinion systems. That assembly, however, is subject to the same reservations noted above in reference to ball screw arrangements. For those reasons it has not been commercially used.
Another variation on the jacking mechanism is known in which a sleeve surrounds the leg and is rigidly attached to the platform, see U.S. Pat. No. 4,007,914 issued to Sutton on Feb. 15, 1977. In that jacking assembly, a pair of movable yokes are arranged with a rack and pinion mechanism to couple the forces exerted on each yoke with the single yoke engaging the leg. While the jacking mechanism operates with short strokes, the actual jacking operation is effected with hydraulic cylinders. Due to practical limits on the effective stroke of an hydraulic cylinder, there has been industry resistance to adopting that jacking assembly.
With the experience gained heretofore by various oil exploration companies there is now a trend to using jackup type platforms as oil production platforms in offshore oil fields. Acccordingly, these jackup platforms may realistically be expected to be in one position for 20 years or more until the particular oil field is depleted. On the other hand, the jackup type rig is traditionally designed as a portable reuseable type of drilling platform. For example, after an oil exploration well has been completed, the platform may be lowered to the floating condition and the legs then lifted so that the platform can be taken to a new position to again commence oil exploration. Thus, very expensive known jacking assemblies required for lifting the platform with respect to the legs, lead to an unnecessarily high capital expenditure for platforms intended to be used for oil production.
It will therefore be apparent that the need continues to exist for a jacking assembly which overcomes problems of the type discussed above.